Referring to FIG. 1, which is a sectional side view, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produces two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
Delivery of fuel within a gas turbine engine is important for achieving operational performance. A number of processes have been utilised in the past to control fuel supply within a gas turbine engine. Mechanical systems use rods and/or a unison ring to distribute and control displacement of fuel valves driven from a remote input drive system which is generally fuel pressure controlled typically using fueldraulic servo-activators. Such mechanical systems suffer since care must be taken with physical alignment and use of appropriate bearings, and thermal growth in the rods and unison ring must be considered. It will also be understood that a relatively large number of external dynamic seals are required for the system. Such seals present considerable fire and reliability problems.
More recently hydraulic control systems using pilot pressure to distribute control of fuel through fuel valves have been proposed. U.S. Pat. No. 6,955,040 and U.S. Pat. No. 7,036,302 provide examples of such hydraulically controlled fuel control arrangements and systems. Unfortunately such hydraulically controlled fuel control systems require considerable additional fuel lines and supplies along with wasted flow dynamics to produce the necessary thermal pressure control and to attempt to reduce lacquering of stagnated fuel or temperature damage to slowly moving fuel. Insufficient control of fuel lacquering can lead to valve functional defects. Furthermore, as there are no mechanical interconnections between the fuel valves at each fuel injector it is difficult to achieve the safety and reliability requirements for a convenient yet fully acceptable system.
A further prior approach to fuel control arrangements relates to utilisation of flexible drive actuation processes to control individual fuel valves by a remote drive system. Such flexible drive actuation systems have advantages but it will be understood that the control devices are located at the fuel injector and so in extremely high temperature environments about an engine core. These environmental considerations do not lend themselves to sensing actual or accurate fuel valve positions and therefore through feedback control loops adjusting necessary valve position for fuel requirements and demand. Furthermore there are system problems typically in relation to assembly and rigging to ensure that the assembly is correctly aligned for desired functionality.